This is a great question and one that each of us confronts every day. To some extent, the answer depends on the type of bulbs you’re using.
Traditional incandescent bulbs are so inefficient, that it really is better to turn them off when they are not needed, even if you’re leaving the room for only a few minutes.
However, with compact fluorescent blubs (CFLs) you can be a little more flexible. The amount of energy used to turn lights off and on again – the “inrush” – is typically equal to only a few seconds of normal light operation. So, technically, you’re saving energy by turning off CFLs.
But, CFLs are so energy efficient that you’re not wasting too much energy by leaving them on if you’re only stepping away for just a few minutes.
A common misperception is that it takes a lot of extra energy to turn on a CFL. This may seem true since some CFLs take a few minutes to “warm up” before they reach full brightness. Turning on a light does require additional energy; however, the bulb’s “warm up” period does not draw extra electricity.
Rule of thumb: If you know you’ll be away for more than a few minutes, you should make a habit of turning off your lights. And, regardless of your habits, you should buy CFLs and install them everywhere. The savings are enormous.
Voila! – a quick, easy way to REDUCE the amount of energy – not to mention the number of light bulbs – you burn through!
By the by, you can submit your own question to Dr. Bill here.